The invention relates to a process for the production of polyalcohols in the form of sugar alcohols from the group of sorbitol and/or mannitol and optionally further C6 and/or C4 and/or C3 and/or C2 polyols.
Sugar alcohols have numerous industrial uses. Thus, e.g. sorbitol is in particular used as a sugar substitute with a sweetening character, e.g. for dietetic foods, in the cosmetic and pharmaceutical industries and also technically in the paper and textile industries. Mannitol is e.g. also used as a sugar substitute, as a filler in the pharmaceutical industry, in the production of synthetic resins, etc. In addition, polyols with up to six carbon atoms, such as propanediols, propanetriols (glycerin) and butanediols are of great technical significance and are e.g. used in the petrochemical industry as a base substance for the production of plastics. At present such polyols are obtained from fossil fuels, particularly petroleum and after fractionation are used for plastic synthesis, such as in the production of polyurethanes. In this connection the term “polyols” also covers organic alcohols with at least two hydroxy groups.
For the production of sugar alcohols biochemical processes are known, in which sugar alcohols are produced by enzymatic treatment of the corresponding monosaccharides, but this requires very long reaction times lasting several hours or days in order to obtain a high sugar alcohol yield.
Thus, sugar alcohols are generally produced by the catalytic hydrogenation of saccharides or other carbonyl compounds with hydrogen under elevated pressure and temperature. Apart from discontinuous or batchwise performed processes in which the reaction mixture is generally stirred over several hours, continuous processes are known in which the reaction mixture is contacted in tubular reactors with the hydrogenating catalyst.
DE 1 002 304 A describes a process for the continuous hydrogenation of reducible sugars, where a fine particular nickel catalyst is suspended, mixed with an aqueous educt solution and then the mixture is continuously contacted under elevated pressure and temperature with a hydrogen flow. The educts used are in particular glucose, inverted cane sugar or lactose, which have been reacted in the aforementioned manner to sorbitol and mannitol (from glucose or inverted cane sugar) or lactosite (from lactose). However, the long reaction times necessary for a satisfactory conversion yield and the relatively limited selectivity for the production of the specific sugar alcohols are disadvantageous.
DE 960 352 B discloses a process for the production of a mixture of the sugar alcohols sorbitol and mannitol, in that an aqueous saccharose solution is hydrolyzed under elevated pressure and temperature in the presence of a nickel catalyst. However, it is disadvantageous that there is a long reaction time of 45 minutes up to several hours necessary in order to achieve a satisfactory hydrolysis reaction yield, the latter being broken off when it is completed in a range between 95% and 99%. Furthermore the selectivity with respect to specific representatives of sugar alcohols is relatively limited.
DE 199 29 368 A1 describes a process for the production of the sugar alcohol mannitol from fructose, the fructose being continuously hydrogenated in the presence of a Raney nickel catalyst at a temperature between 50° and 180° C. and a pressure between 50 and 300 bar. Fructose is used in aqueous solution and hydrogen is used as the hydrogenating agent. No information is given on the reaction time necessary for a satisfactory educt reaction, but here again the selectivity is relatively limited and in particular exclusively mannitol and sorbitol mixtures can be produced.
DE 100 65 029 A1 relates to a similar process for the production of alcohols, particularly sugar alcohols, by reacting carbonyl compounds, particularly sugars, such as sorbitol from dextrose, sorbitol and mannitol from fructose, xylite from xylose, maltite from maltose, isomaltite from isomaltose, dulcite from galactose and lactite from lactose. The educts are continuously reacted with hydrogen in the presence of a Raney catalyst based on nickel, cobalt, copper, iron, platinum, platinum, palladium or ruthenium in aqueous solution. A pressure between 30 and 450 bar and a temperature of max. 150° C. are set, in order in the case of the use of sugars as educts to ensure that the latter do not caramelize. With regards to the production of sugar alcohols, no details are given on the reaction time or selectivity and as a result of the indicated caramelization risk at temperatures above 150° C. and a multistage hydrogenation relatively long reaction times must be assumed.
DE 1 931 112 A1 describes a process for the production of mannitol and sorbitol from saccharose, in that a saccharose solution is hydrogenated with hydrogen in the presence of a hydrogenating catalyst at a temperature of approximately 160° to 190° C. and a pressure of approximately 35 to 211 bar, the process being continuously performable. Metal catalysts based on nickel are referred to as hydrogenating catalysts. The residence time is 15 minutes to 2.5 hours.
EP 773 063 A1 discloses another process for hydrogenating pure sugar alcohols using a hydrogenating catalyst in the form of Raney nickel. The process parameters indicated are e.g. temperatures of 110° to 150° C. and pressures of 40 to 200 bar. According to an embodiment crystalline glucose is hydrogenated at 130° C. and 150 bar in a continuous Raney nickel flow through a reactor, the sorbitol yield obtained being 99.3%. The use of a catalyst based on ruthenium/ruthenium oxide is not mentioned, because when performing the process in accordance with EP 773 063 A1 this allegedly leads to isomerization, decomposition and polymerization during hydrolysis.
The problem of the invention is to further develop a process for the production of polyalcohols of the aforementioned type in such a way that in the case of a high selectivity for the desired sugar alcohols, particularly sorbitol and/or mannitol, as well as optionally for further C2 to C6 polyols, it is economic and effective and in particular permits a simple control of the selectivity with respect to the in each case desired products.